For describing the background of the invention, particular reference is made to the following publications:    [1] A. H. Zewail in “J. Phys. Chem.” A 104, 5660-5694 (2000);    [2] F. Krausz et al. in “Rev. Mod. Phys.” 81, 163-234 (2009);    [3] S. R. Leone et al. in “Nat. Photonics” 8, 162-166 (2014);    [4] G. Sansone et al. in “Nat. Photonics” 5, 655-663 (2011);    [5] D. Shorokhov et al. in “J. Chem. Phys.” 144, 080901 (2016);    [6] A. Gordon et al. in “Opt. Express” 13, 2941-2947 (2005);    [7] T. Popmintchev et al. in “Science” 336, 1287-91 (2012);    [8] H. Fattahi et al. in “Optica” 1, 45-63 (2014);    [9] A. Wirth et al. in “Science” 334, 195-200 (2011);    [10] C. Manzoni et al. in “Opt. Lett.” 37, 1880-1882 (2012);    [11] A. Moulet et al. in “Opt. Lett. 39, 6189 (2014);    [12] H. Fattahi, Third-Generation Femtosecond Technology, Springer Theses (Springer International Publishing, Cham, 2015);    [13] F. Silva et al. in “Nat. Commun.” 3, 8071-8075 (2012);    [14] S. Keiber et al. in “Nat. Photonics” 10, 159 (2016);    [15] H. Fattahi et al. in “Opt. Lett.” 41, 1126-1129 (2016);    [16] T. Buberl et al. in “Opt. Express” 24, 10286 (2016);    [17] J. C. Travers et al. in “J. Opt. Soc. Am.” B 28, All (2011);    [18] O. D. Muecke et al. in “IEEE J. Sel. Top. Quantum Electron. 21, 1-12 (2015);    [19] M. Schultze et al. in “Opt. Express” 18, 27291-97 (2010);    [20] H. Fattahi et al. in “Opt. Express” 20, 9833-40 (2012);    [21] A. Schwarz et al. in “Opt. Express” 20, 5557-5565 (2012);    [22] U.S. Pat. No. 8,599,889 B2; and    [23] U.S. Pat. No. 9,244,332 B1.
Femtosecond laser technology enabled fs time resolved spectroscopy and new branches of science, like femtochemistry [1] or four-dimensional imaging [5]. The availability of 100 eV-attosecond pulses one decade ago [2] pushed the time resolution of pulses, in particular in the XUV and towards the X-ray wavelength range to attosecond and allowed the study of electron's motion in the atomic time scale by means of near-infrared-pump, attosecond-probe spectroscopy [3]. However the limited flux of the attosecond pulses of the current technology makes attosecond-pump, attosecond-probe spectroscopy nearly impossible and calls for sources with higher photon flux [4].
As the high harmonic generation (HHG) used for generating pulses has a cutoff energy, which scales linearly with the peak intensity and quadratically with the wavelength of the driving pulse, two routes can be considered to extend as pulses to the X-ray regime: i) pushing the central wavelength of the deriving laser to longer wavelength in expense of the lower photon flux of the generated harmonics [6, 7] or ii) scaling the peak-power of the driving laser source [8].
Short pulses available from the current laser technology contain more than one optical cycle of the electric field. Multi-cycle pules at high peak intensities tend to pre-ionize the atoms before the arrival of the main peak of the electric field, responsible for the generation of the highest cutoff energy in HHG. This barrier can be overcome by using tailored single-cycle pulses [9] to drive the HHG. A pulse source for pushing the frontiers of attosecond technology should not only fulfill the above criteria, but also be able of delivering laser pulses at tens or hundreds of kHz repetition rates.
Combining the concept of the field synthesize [9, 10] with Yb-doped-pumped optical parametric chirped pulse amplifiers (OPCPA) provides the possibility of generation of tailored sub-cycle light transients at longer wavelength and higher peak and average-power [8]. It is expected that these high-energy light transients may lead to the new generation of as pulses, extending the HHG cutoff energy to keV, X-ray pulses [8, 11] and providing a platform for as X-ray four dimensional imaging.
The architecture of an optical field synthesizer based on Yb:YAG thin-disk lasers is studied theoretically in [8, 12]. The apparatus comprises (i) an Yb:YAG thin-disk regenerative amplifier, followed by (ii) phase-stable multi-octave seed generation, (iii) amplification of three complementary portions of the supercontinuum (SC) in OPCPA channels, and (iv) coherent recombination of the output of these channels.
It is import to seed the OPCPA chains of the optical field synthesizer with a CEP stable supercontinuum, as waveform synthesis enables control over the electric field of femtosecond pulses on a sub-cycle scale. Additionally, the direct generation of the superoctave seed pulses from the OPCPA pump source assures minimum temporal jitter between pump and seed pulses in the amplification chains. However, direct CEP stable SC generation from 1-ps pulses has been challenging so far.
It is generally known that CEP stability can be obtained by active or passive CEP stabilization techniques. Active CEP stabilization uses fast feedback loops controlling the pulse dynamics of a seed pulse source laser oscillator. The active control has disadvantages in terms of complexity, reliability and stability. With passive CEP stabilization, CEP-stable seed-pulses for the signal channel and seed-pulses for the pump pulse channel in OPAs and/or OPCPAs are generated on the basis of the same driving pulses, resulting in advantages in terms of increased reliability.
For example, with the passive CEP stabilization according to [22], three outputs of a sub-ps pump source are used for generating CEP stable broadband pulses by difference frequency generation (DFG) and for seeding the pump line for OPA or OPCPA based amplification. This technique has disadvantages as the pump source is restricted to create short pulses with a duration below 500 fs, so that the bandwidth of the generated pulses is restricted. Furthermore, the central wavelength of the generated short pulses can be only longer than the pump wavelength. The two pulses send to the DFG stage are restricted to short, intense pulses, so that Yb:YAG thin-disk lasers cannot be used as the pump source.
Another passive CEP stabilization is suggested in [23], wherein the CEP stability is not obtained by a DFG stage like in [22], but by a chain of amplification stages driven by an Yb based picosecond (ps) pump source. The amplification stages are driven by narrowband pulses derived from the ps pump source in [23]. As the CEP stable SC generation is based on a CEP-stable idler beam of the amplification stages, the amplification stages have to be used in a collinear geometry to avoid the angular chirp on the idler. This approach limits the achieved CEP-stable pulses to a narrow wavelength range only, so that the technique of [23] can have limitations in terms of restricting the SC generation to the driving with an idler beam having a fixed central wavelength.
Further to the application in optical field synthesizers, a need for a CEP-stable fs pulsed light source exists in other fields, e. g. time-resolved spectroscopy.